A slow growing nitrogen-fixing strain ofVigna radiata var.aureus (mung bean)Rhizobium which expressed nitrogenase activity in a synthetic medium was isolated from its native population. Mutants with decreased and increased nitrogenase activity were derived from this strain by treatment with acridine orange and ethidium bromide. These mutants were tested for symbiotic effectivity invivo. The effectivity of mutants with decreased nitrogenase activity in the culture medium was lower than the parent strain; however, the effectivity of mutants with higher nitrogenase activity did not increase above that of the parent. This suggests that the plant is perhaps a limiting factor in the full expression of rhizobial nitrogenase in the nodules.

Azospirillum brasilense, an associative diazotrophs from sorghum roots grows autotrophically on NH4+ and CaCO3. NH4+ a is also oxidized to NO2- and then denitrified. Addition of malate to the autotrophic medium enhances both NH4+ oxidation as well as NONH2- dissimilation. The incomplete nitrification linked denitrification results in a rapid loss of nitrogen from the growth medium. The bacterium also shows assimilatory NO3- and NO2- reductases and fixes nitrogen at 50 μg N/ml of NH4+ NO3- or NO2-

Uptake hydrogenase activity in nodules of green gram (Vigna radiata (L.) (Wilczek)), black gram (Vigna mungo (L.) (Hepper)), cowpea (Vigna unguiculata (L.) and cluster bean (Cyamopsis tetragonoloba (L.) (Taub.)), formed with two Hup+ (S24 and CT2014) and one Hup− (M11)Rhizobium strains, was determined at different levels of external H2 in air atmosphere. Nodules of all the 4 host species formed by inoculation with strains S24 and CT2014, showed H2 uptake but not those formed with strain M11. H2 uptake rates were higher in 1 and 2% H2 in air atmosphere (v/v) than at 5 or 10% levels in all the host species. Variations in the relative rates of H2 uptake were observed both, due to host species as well as due toRhizobium strains. However, no host dependent complete repression of the expression of H2 uptake activity was observed in nodules of any of the host species formed with Hup+ strains.